The present invention relates to a process for forming semiconductor structures using activated process gases.
Integrated circuits have bipolar transistors or Metal Oxide Silicon Field Effect Transistors (MOSFET) that contain conducting, dielectric, and semiconducting layers deposited or thermally grown on a semiconductor substrate by chemical and physical vapor deposition methods. Photolithographic methods are used to form a patterned layers of mask and/or photoresist features covering the layers on the substrate, and the exposed regions of the layers are etched by activated or energized gases, such as for example, CF4/O2, CF4/NF3, NF3/He, and CHF3/CF4/Ar. One problem with these processes arises during removal of residual photoresist from the substrate by ashing or stripping processes. Conventional ashing or stripping processes utilize an oxygen plasma at temperatures greater than 200xc2x0 C. to ash the residual photoresist. Although these processes are highly selective in stripping photoresist, the high temperatures damage the active devices on the substrate by causing increased diffusion of mobile ions. It is desirable to have a resist removal process that ashes resist without causing diffusion of ions and other materials in the substrate.
Another problem arises from the cleaning or removal of polymeric etchant deposits that are formed on the substrate. The polymeric etchant deposits are byproducts of condensation reactions that are formed between vaporized metal, silicon, and resist species which include carbon, hydrogen, oxygen, and nitrogen. The polymeric etchant deposits are difficult to remove from the substrate because of their chemical composition. Furthermore, because the composition of the polymeric etchant deposits varies depending on the material being etched, the resist composition, and the composition of the etching gas, it is often difficult to clean all of the polymeric etchant deposits without excessive etching of the underlying silicon or other layers.
Yet another problem arises because xe2x80x9cnative silicon dioxidexe2x80x9d films are formed on the exposed silicon containing layers on the substrate, especially during processing of MOSFET structures. For example, oxygen plasmas that are used to ash residual resist on the substrate can cause a native silicon dioxide film to form on exposed silicon portions of the substrate. The native silicon dioxide film typically comprises a layer of silicon dioxide in a thickness of from about 10 to 20 xc3x85, that is formed by oxidation of silicon at elevated temperatures in a process gas containing oxidizing species or oxygen-containing gases. Silicon dioxide films are electrically insulating and are undesirable at interfaces with contact electrodes or interconnecting electrical pathways because they cause high electrical contact resistance. For example, in MOSFET structures, the electrodes and interconnecting pathways comprise self-aligned silicide layers that are formed by depositing a refractory metal layer on bare silicon and annealing the layer to produce a metal silicide layer. Native silicon dioxide films at the interface between the substrate and refractory metal reduce the compositional uniformity of the silicide layers by impeding the diffusional chemical reaction that forms the metal silicide layers. This results in lower substrate yields and increased failure rates due to the overheating at the electrical contacts. The native silicon dioxide film can also prevent adhesion of CVD or sputtered layers which are subsequently deposited on the substrate.
Various methods have been used to etch or clean the native silicon dioxide layers formed on the substrate. However, it is difficult to remove the thin native silicon dioxide layers without damaging the underlying or surrounding layers. For example, U.S. Pat. No. 5,022,961 discloses a process for removing a native silicon dioxide film by etching the substrate with energized gaseous anhydrous hydrogen fluoride and alcohol; and U.S. Pat. No. 4,749,440 describes a similar method using a plasma of hydrogen fluoride and water vapor. The high fluorine content reacts with the native silicon dioxide film to form gaseous SiF4 byproducts, and the alcohol forms a layer covering the silicon surfaces that prevents regrowth of the native silicon dioxide film. However, in both cases an excessive concentration of fluorine ions remain on and contaminate the substrate surface. The energetic plasma etching processes are also undesirable because the energetic impingement of the plasma ions on the substrate often damages the silicon lattice structure of the exposed silicon surfaces on the substrate. The bombardment by the plasma ions creates lattice defects and dislocations that increase junction leakage. In addition, the high etch rates produced by conventional etching methods are difficult to control when etching the thin native silicon dioxide layers on the substrate and can cause excessive etching of surrounding or underlying silicon-containing layers. Also, etchant halogen ions can contaminate the substrate, requiring an additional water cleansing process step to remove the contaminants. This additional process step lowers the throughput of the etching process, and the high surface tension of the water can cause the cleansing step to become ineffective in removing contaminants from trenches or holes having high-aspect ratios.
Accordingly, there is a need for a process for selectively removing residual resist, polymer etchant deposits, and native silicon dioxide films without damaging or etching surrounding or underlying silicon-containing layers. It is also desirable for the process to remove damaged portions of the silicon lattice structure. It is further desirable to remove native silicon dioxide films with high removal rates, good uniformity, and high selectivity ratio relative to the surrounding silicon-containing layers. Finally, it is desirable for the process to provide low levels of contamination of the substrate by contaminant halogen ions.
The present invention provides a method for removing polymeric etchant deposits, silicon lattice damage, and native silicon dioxide layers on a substrate. In the method, the substrate is placed in a process zone, and exposed to an activated cleaning gas comprising inorganic fluorinated gas and oxygen gas, to remove polymeric etchant deposits o the substrate. Thereafter, the substrate is exposed to an activated etching gas to remove silicon lattice damage on the substrate. The substrate is then exposed to an activated reducing gas to remove a native silicon dioxide layer on the substrate. The cleaning gas, the etching gas, and the reducing gas can be activated remotely or in situ by microwaves or RF energy. Preferably, the activated gases are activated in a remote chamber that is remote from the process zone.
The present invention is particularly useful for a method of forming an active electronic device on a substrate. In the method, a dielectric layer on the substrate comprising one or more of silicon oxide or silicon nitride is etched by exposing the substrate to a dielectric etching gas. Thereafter, polymeric etchant deposits on the substrate are removed by exposing the substrate to activated cleaning gas comprising oxygen gas and inorganic fluorinated gas in a first volumetric flow ratio. The substrate is then exposed to an activated etching gas comprising oxygen gas and inorganic fluorinated gas in a second volumetric flow ratio. An activated reducing gas comprising a hydrogen-containing gas removes native silicon dioxide layers by reacting hydrogen species with oxygen in the native silicon dioxide layer to form volatile water vapor, which is exhausted from the chamber, leaving behind elemental silicon. A metal layer is then deposited on the silicon layer on the substrate thereby exposed, and the substrate is annealed to form a metal silicide layer.
In another aspect, the present invention is directed to a method of removing a native silicon dioxide layer on a substrate by reducing the layer using a reducing gas comprising a hydrogen-containing gas. In the method, the reducing gas is introduced into a remote zone that is remote from the process zone, and a microwave-activated reducing gas is formed by coupling microwaves in the remote zone. Then the microwave-activated reducing gas is introduced into the process zone to reduce the native silicon dioxide layer to a silicon layer. The reducing gas comprising hydrogen-containing gas and inorganic fluorinated gas is capable of forming activated HF species that reduce the native silicon dioxide layer. Preferably, the hydrogen-containing gas comprises one or more of H2, CH3F, H2S, NH3, CH4, C2H6, C3H8, and B2H6. More preferably, the reducing gas comprises inorganic fluorinated gas and hydrogen-containing gas in a volumetric flow ratio of from about 1:5 to about 5:1.